Microstructure and dielectric properties of biocarbon nanofiber composites
© Dai et al.; licensee Springer. 2013
Received: 19 April 2013
Accepted: 7 June 2013
Published: 22 June 2013
A kind of web-like carbon with interconnected nanoribbons was fabricated using bacterial cellulose pyrolyzed at various temperatures, and the microwave dielectric properties were investigated. Bacterial cellulose was converted into carbonized bacterial cellulose (CBC) with a novel three-dimensional web built of entangled and interconnected cellulose ribbons when the carbonization temperature was below 1,200°C; the web-like structure was destroyed at a temperature of 1,400°C. Composites of CBC impregnated with paraffin wax exhibited high complex permittivity over a frequency range of 2 to 18 GHz, depending on the carbonization temperature. Both real and imaginary parts were the highest for CBC pyrolyzed at 1,200°C. The complex permittivity also strongly depended on CBC loadings. For 7.5 wt.% loading, the real and imaginary permittivities were about 12 and 4.3, respectively, and the minimum reflection loss was -39 dB at 10.9 GHz. For 30 wt.% loading, the real and imaginary permittivities were about 45 and 80, respectively, and the shielding efficiency was more than 24 dB in the measured frequency range and could be up to 39 dB at 18 GHz. The electromagnetic properties were assumed to correlate with both the dielectric relaxation and the novel web-like structure.
The rapid proliferation of advanced electronic devices for many commercial and military applications, such as data transmission, telecommunications, wireless network systems, and satellite broadcasting as well as radar and diagnostic and detection systems, has led to numerous electromagnetic compatibility and electromagnetic interference (EMI) problems. The interaction of electromagnetic waves originating from different sources can lead to a decrease in quality and a misinterpretation of transferred data, and it has thus become vital to avoid such interference and electromagnetic wave pollution through the use of appropriate absorbing and shielding materials. Carbonaceous materials - such as graphite and/or carbon black - are often used as dielectric electromagnetic absorbers, generating dielectric loss by improving the electrical conductivity of the mixture. In particular, nanostructured materials and carbon fiber composites have been the subjects of growing interest as microwave radiation absorbing and shielding materials in the high-frequency range due to their fascinating properties [1–5]. It is reported that carbon nanofiber-polymer composites presented EMI shielding effectiveness (SE) of approximately 19 dB with 15 wt.% carbon nanofiber loading . Graphite-coated FeNi nanoparticles exhibited reflection loss (RL) of approximately -23 dB with the thickness 2.5 mm and the absorption peak at 14 GHz . Carbon nanocoils coated with Fe3O4 exhibited remarkably improved microwave absorption (RL approximately -20 dB) compared to the pristine carbon nanocoils (RL approximately -2 dB) . Another allotrope of carbon, viz., single-layered two-dimensional graphene, graphene oxide, or reduced graphene oxide, has attracted a great deal of attention for its application in many diverse areas due to its unique electrical, mechanical, and thermal properties in addition to its light weight, high surface area, and layered morphology. The graphene/epoxy composites exhibited SE of approximately 21 dB in the X-band for a 15 wt.% loading . The reduced graphene oxide exhibits -7 dB RL while graphite only exhibits approximately -1 dB in the frequency range of 2 approximately 18 GHz . Further to the considerable interest in adding small concentrations of nanocarbons into the matrix, what unquestionably matters is the ability to disperse them . The cost and limited supply also hinders the application of nanocarbons as fillers for EMI shielding and microwave absorption. Recently, researchers have tried low-cost natural materials (rice husks) as carbonaceous sources to fabricate carbon-matrix composites with self-assembly interconnected carbon nanoribbon networks . These composites have higher electric conductivities and EMI shielding effectiveness values than those without. In this paper, the example of microwave composites is reported using bacterial cellulose as the carbonaceous source, which had self-assembled interconnected nanoribbon networks. These composites exhibited high permittivity in the frequency range of 2 to 18 GHz and thus could be excellent high-loss materials, for example, as an EMI material or high-performance microwave absorbing material. The interesting electromagnetic characteristics are due to the novel three-dimensional web-like networks which establish additional electrical conduction pathways throughout the whole system.
Carbonized bacterial cellulose (CBC) was obtained by heat-treated bacterial cellulose (BC), which was pyrolyzed for 4 h under a nitrogen atmosphere at 800°C, 1,000°C, 1,200°C, or 1,400°C. CBC was cleaned using diluted hydrochloric acid with volume fraction of 10% and then soaked in concentrated nitric acid at room temperature for 4 h. Afterwards, the black solution was diluted with distilled water and rinsed for several times until the pH value reaches 7. The resulting CBC were separated from the solution by filtration and dried using a vacuum at 60°C for further use. Dried CBC fibers were mechanically milled into powder for the measurement of electromagnetic parameters. The CBC/paraffin wax samples were prepared by uniformly mixing the powders in a paraffin wax matrix. A series of CBC/paraffin wax composites were prepared with CBC loading of up to 30 wt.%. The absorbers were dispersed in ethanol with paraffin wax by stirring and sonication at 90°C for 1 h. The mixtures were then pressed into cylindrical dies with 7.0 mm outer diameter, 3.0 mm inner diameter, and about 2.0 mm height.
The morphology of CBC was observed by transmission electron microscopy (TEM, Tecnai F20, FEI, Hillsboro, OR, USA) and scanning electron microscopy (SEM, FEI NOVA600i). The sheet resistance (Rs) of the composites was measured by the four-probe method using a Keithley 2400 multimeter (Cleveland, OH, USA), and the direct current (DC) conductivity σ was obtained using the measured Rs and the sheet thickness t according to σ = 1/(Rst). Complex permittivity and permeability measurements were performed on an Agilent E8363B vector network analyzer in the 2 to 18 GHz frequency range. Three samples were tested for each electromagnetic parameter measurement, and the reported results are the averages.
Results and discussion
Phase and microstructure of CBC
Microwave electromagnetic properties of CBC
For EMI shielding, the total shielding effectiveness SE T is always expressed by SE T = 10 lg(Pin/Pout) = SE A + SE R + SE I , where Pin and Pout are the power incident on and transmitted through a shielding material, respectively. The SE A and SE R are the absorption and reflection shielding efficiencies, respectively, and can be described as SE A = 8.686 αt and SE R = 20 lg |1 + n|2/4|n| . For the composite with 30 wt.% CBC pyrolyzed at 1,200°C, using the measured electromagnetic parameters, we calculated the SE A (with the thickness t assumed to be 2.0 mm) and the SE R , which together with SE A + SE R are shown in Figure 6b. It can be seen that the SE T increased from 24 dB in the low frequencies to 39 dB at 18 GHz. The contribution to the SE T was mainly from the reflection in the low frequency range and from the absorption in the high range. The EMI shielding efficiency is attributed to the formation of conducting interconnected nanofiber networks in an insulating paraffin wax matrix that will interact with the incident radiation and lead to the high shielding effectiveness.
The pyrolysis of bacterial cellulose led to the formation of a unique interconnected web-like network of carbon nanoribbons, and this was used to fabricate carbon-matrix composites. These composites had remarkable imaginary permittivities and huge loss tangents and thus good attenuating properties. The web-like networks were very helpful for increasing the dielectric loss. The electromagnetic properties could be optimized by manipulating the bacterial nanoribbons by doping or surface modification; and thus, the RL and SE T could be further improved. Based on these properties, and taking into account its other advantages, such as its light weight, easy processability, high mechanical strength, and good dispersion in the matrices, such CBC has the potential to be as an effective EMI shielding material and microwave absorber.
We thank Prof. C. H. Pei for the helpful discussions and Dr. J. S. Liu for the technical assistance. This work was supported by the National Basic Research Program of China (no. 2011CB612212), the Program for New Century Excellent Talents in University (no. MCET-11-1061), and the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (no. 11zxfk26) of China.
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